The invention relates to a method of preparing polyvinyl chloride and copolymers of vinyl chloride by polymerization is seeded microsuspension. It also relates to the resultant polyvinyl chloride and copolymers.
Polymerization in microsuspension, or fine suspension, is understood as being polymerization, in the presence of initiators soluble in organic material, or at least one monomer dispersed by powerful mechanical means in an aqueous medium containing an emulsifier as stabilizer, in order to obtain a dispersion of particles of an average diameter less than 5 microns.
It is known, from French Pat. No. 1,485,547, to prepare polymers of vinyl chloride in microsuspension, in the presence of a seeding product containing all the initiator necessary for polymerization. However, when polymerization is carried out so as to obtain a latex with a polymer concentration of over 45%, the high viscosity of the medium necessitates slower agitation, owing to the mechanical instability of the latex formed. This slow agitation leads to a decrease in the coefficients of exchange, which means that the polymerizing time has to become longer.
Moreover when the polymers obtained are used to prepare plastisols, the plastisols are difficult to use because of their dilatance, i.e. the increase of their viscosity with the speed gradient, and because the minimum proportion of plasticizer that can be used is 50 parts per 100 of polymer.
It is also known that, in the case of polymers of vinyl chloride prepared by polymerization in emulsion, the use of a plastisol is improved if the polymer comprises elementary particles forming at least two populations which have particles of different sizes. This plurality of populations can be obtained by mixing several latices comprising particles of different sizes, but the mixture cannot be made at a high concentration. Another solution consists of simultaneously preparing the various populations of particles by polymerization in emulsion of at least one monomer in the presence of several seeding products prepared by polymerization in emulsion. However, this operation is very tricky, for the quantity of emulsifier used has to be stipulated and limited and its introduction requires special care, that is to say, a specific introduction all the time, in order to avoid nucleation of new particles which would limit the polymer concentration of the latex obtained. Moreover it is difficult to make seeding products with particles of very small sizes which are measured and homogeneous, since the slightest variation in particle size results in variation in the ratios of the dimensions and in the proportions of the various populations obtained after polymerization.
The method of the invention avoids these drawbacks and enables latices to be prepared by polymerization in microsuspension, without any limitations, precautions or difficulties; the latices having a polymer concentration well over 45% by weight, a low viscosity and a reproducible particle size. Furthermore, the polymers thus obtained are easy to convert and more particularly adapted to give plastisols which may contain very little plasticizer but which are nevertheless easy to use.